The protozoan parasite Cryptosporidium is increasingly recognized as an important cause of sporadic diarrheal disease in the normal host especially children and of life-threatening diarrheal disease in patients with compromised host defenses including cellular and humoral immunity. In addition, Cryptosporidium is emerging as both an important zoonosis and cause of diarrhea in the developing world. There is no available effective therapy for this infection currently. The mechanism(s) by which this parasite induces pronounced intestinal secretion in susceptible hosts has not been studied. A systematic study of the effect of Cryptosporidium infection on intestinal function and investigations to identify the intracellular mediators important to the secretory response observed will advance our understanding of the regulation of intestinal ion transport and allow development of therapeutic strategies aimed at interrupting the profound physiologic effects of Cryptosporidium infection. In order to investigate the alterations in gastrointestinal physiology associated with Cryptosporidium infection, we will establish a symptomatic neonatal piglet model of infection using a quantitated innoculum. In vivo studies will be performed to carefully characterize the natural history of the infection and its effects on protein, fat and carbohydrate absorption. Mucosal dissacharidase levels, and thymidine kinase will be used to assess mucosal function and the rate of mucosal epithelial cell turnover. In vitro studies in Ussing chambers will identify the specific ion fluxes altered by infection with Cryptosporidium in functionally distinct areas of bowel. Assay of Na+/K+ ATPase activity of mucosal cells, and the response of infected tissues to glucose in vitro in Ussing chambers will identify if a defect in glucose-coupled sodium transport occurs in intestinal infection with Cryptosporidium. Known pathways mediating intestinal secretion will be explored as potential mechanisms by which Cryptosporidium causes diarrhea. Detailed studies will be designed to dissect the role of cyclic nucleotide dependent pathways, calcium dependent mechanisms and pathways stimulated by the active products of arachidonic acid metabolism. Identification of important intracellular messengers will enable the evaluation of both antisecretory pharmacologic agents and agents promoting absorption as potential approaches to the management of disease caused by this currently untreatable, but potentially devastating, diarrheal pathogen.